I have a LP/electric fridge in the 5er and have been off grid for over a month at a time without having to refill a propane bottle. Frig use of the LP is VERY minimal. As for the LP HW, if off grid I only turn it on about 30 minutes before showering or doing dishes, then off. Never had an issue running out of propane except when running the furnace. My 5er has a gasoline 5.5 Onan and generally about 2 hours a day keeps dual 12v battery bank up. We do that in the morning, while making coffee, and in the evening while using the microwave. When possible, we make that concurrent with need HW so that water is heated electrically.
I don't have a tankless HW heater so can not speak from experience, but based on the posts I've seen, even when acceptable there is still more water use/waste than with the tank models. Fresh water and holding tank levels are critical when you are off grid.
May have to settle for the residential fridge to get a coach I would have, but that means 2 more batteries, replaced every 2.5 to 3 years at $100 each, OR, higher end batteries every 5 to 6 years at $200 each, plus you NEED auto-start for your generator and that's another, I would guess, $500 or so, unless you have a higher dollar coach that comes with it.
If you're doing most of your "camping" with hookups I see no problem with tankless H/W, other than those posted in several hundred posts, and residential fridge. But, we have always enjoyed the off grid parks and prefer not to be confined, or concerned, about campground facilities, other than enjoying the beauty of the setting. I don't have a problem with those who prefer tankless and residential, just with the manufacturer's who don't allow alternatives.
We also have a Sunseeker "C". Have had Jayco's in the past (a little better quality IMHO) but they've gone tankless on all Greyhawks and class "A's", and thus crossed off our list.